A current objective in hematology is the largely selective elimination of pathogenic blood or plasma constituents by methods that upset the body as little as possible. For example, a known method of separation is generally performed by passing heparinized blood through the first filter of a cascade consisting of several filters into a stream containing the corpuscular components and a plasma stream, subjecting the latter to a purification process, recombining the purified plasma and the stream containing the corpuscular particles and reinfusing the recombined blood into the patient.
This known process can be carried out in an extracorporeal blood circulation or in stored blood, through the application of an arrangement utilizing the filter cascade for the purpose of the immediate elimination of pathogenic and/or toxic blood or plasma constituents. In the case of stored blood, the blood plasma may also be separated by ultracentrifuge from the corpuscular blood constituents.
The passage of the plasma constituents through the known state of the art membranes is largely dependent on the rate of flow of blood, the transmembrane pressure and the pore size of the membrane used. At present, capillary membrane filters with a pore diameter of approximately 0.2 to 0.5 .mu.m are suitable to ensure a plasma stream adequate for this process. Capillary membrane filters with a pore size of 0.07 to 0.2 .mu.m and filter areas of approximately 0.6 m.sup.2 are also known, and these may be used as second or third filters of a cascade.
However these known capillary membranes for the treating of the produced plasma stream are disadvantageous in their tendency to form the so-called "secondary membranes" due to protein deposits on their surface. This lowers the membrane permeability rapidly, thereby necessitating the replacement of at least some of the filter element because of the pore occlusion caused by the macroaggregates.
Attempts have been made at keeping the membranes open by pulsating rinses with saline solution during the filtration process, which requires briefly stopping the filtering process and the pumping of rinse solution from a storage vessel through an additional pump and through the filter (H. von Baeyer et al., Trans. Am. Soc. Artif. Intern. Organs 1983: 739). The overloading of the separating system with rinse solution, however, places a great strain on the patient since he receives this solution together with the infusion.
In Applicant's German Offenlegensschrift DE-P 33 10 727.0, a new concept is described for the use of filter candles in plastic containers for the separation of precipitated blood or plasma constituents in extracorporeal circulation systems. The filter candles are characterized by large membrane surfaces at low filling volumes, with pore diameters of the filter candle membranes between 0.2 and 10 .mu.m. This type of system, which utilizes different filters having different exclusion limits for a filter cascade suitable for the separation of precipitated plasma constituents, is also disadvantageous in that a large part of the patient's blood circulates in the purification cycle and is therefore withheld from the patient's circulation, which, of course, leads to a great strain on the patient. In addition, such a system is unsuitable for the elimination of dissolved plasma constituents.
The combination of different filter units creating a filter cascade in one container makes sense in theory as it reduces the circulating extracorporeal blood volume and is more cost efficient. However, there are limits imposed by the construction on this type of arrangement when using capillary filters since the capillaries are gathered in bundles and cast in polyurethane at the ends. After the polyurethane has set, the fibers are cut off at one end so that they are open at the face side. This makes the combination of several fiber bundles in one container impossible since each individual capillary fiber bundle must be sealed off against the other.
The sealing of each capillary fiber bundle at one side was proposed (Gurland ISAO 1983, Kyoto) as a solution to this problem. This arrangement is disadvantageous in that it can be produced only by complex multistep procedures and clogging of the stream occurs very quickly in the capillaries' dead zone since the filling volume of the capillaries is very low.
Thus, there exists a need for an effective, relatively simple means by which to separate pathological and/or toxic species from the blood, plasma or serum including dissolved plasma or serum constituents without the development of the so-called secondary membranes which lower membrane permeability.